RULES OF THUMB FOR STRUCTURAL STEEL DESIGN

1 6 RULES OF THUMB FOR STRUCTURAL STEEL DESIGN . By Craig W. Bursch, PE. When it comes to STRUCTURAL DESIGN , many architects I've spoken 4. 12 inches isn't always 12 inches. with have expressed the desire for a rule of THUMB that might One common misconception is that all STEEL members have an actual lead to more efficient DESIGN for their sub-consultants. In response depth equal to their nominal depth. This is true with angles, STRUCTURAL to their inquiries, I am offering the following guidelines for sizing tubes (also known as hollow STRUCTURAL sections) and channels. An STRUCTURAL STEEL . HSS 12x12 is truly 12 by 12 and a C12 is truly 12 deep. In contrast, a wide-flange STEEL beam, also known as an I-beam or a W-shape, can 1.

2 How deep is a foot? vary. For instance, the most commonly used W12 beams vary from 11. For purlins, calculate 1/2 inch of depth for every foot of span. 7/8 to 12 actual depth. The heaviest W12 section is 16 7/8 deep. (If a STEEL purlin must span 24 feet between girders, it should be at least 12 deep.) For girders, calculate 3/4 inch of depth for every 5. Repetition, Repetition, Repetition. foot of span. (If a STEEL girder must span 24 feet between columns, The more regular the column layout, the more economical the it should be at least 18 deep.) project becomes. The economy comes from greater repetition of beam sizes, column sizes, and connections, which leads to more efficient 2. Deeper is cheaper. fabrication and installation.

3 All other things being equal, a deeper wide-flange beam will be more economical than a shallower one. There are exceptions to 6. Make the right connection. this rule, but it is generally correct. STRUCTURAL STEEL connections generally only account for 1% of the overall STEEL budget. However - especially when dealing in renovation 3. Cantilever one-to-three . work - the connection requirements can sometimes drive the member Cantilevered STEEL beams are commonly used to support sizing. Be aware of specific non-standard conditions that may require architectural features, giving the illusion of an unsupported or extra thought in the connection DESIGN . flying edge. How far is too far for a cantilever to stick out?

4 Gen- erally speaking, if a cantilever exceeds 1/3 of the total backspan, No rule of THUMB can be applied to all situations, but remembering economy is lost and may lead to DESIGN difficulties. So if your these will help achieve a more efficient preliminary layout. Your friendly beam has a 30' backspan, try to keep an adjacent cantilever to less neighborhood STRUCTURAL engineer is an expert in STRUCTURAL STEEL DESIGN , than 10' long. so when in doubt, don't hesitate to call with questions. Craig is a STRUCTURAL engineer in MBJ's Duluth, Minnesota, office. This article was published in the May 2006 issue of Matrix, the of- ficial newsletter of the American In- stitute of Architects Minnesota.